Summary:
Originally, the manual flush calls in db_stress flushes only a single column
family, which is not sufficient when atomic flush is enabled.
With atomic flush, we should call `Flush(flush_opts, cfhs)` to better test this
new feature. Specifically, we manuall flush all column families so that
database verification is easier.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4608
Differential Revision: D12849160
Pulled By: riversand963
fbshipit-source-id: ae1f0dd825247b42c0aba520a5c967335102c876
50895e5f0d